Petroleum is recoverable from subterranean formations in which it has accumulated only if certain essential conditions exist; namely, the formation must have adequate permeability or interconneted flow channels so a fluid will flow from one portion of the formation to the other if a pressure gradient is applied to the formation; the petroleum viscosity must be sufficiently low that it will move if flow channels exist and the pressure differential is applied to the fluid, and finally a source of energy to provide pressure differential for causing fluid movement in the formation must exist naturally or be supplied to the formation. When all three of these conditions co-exist naturally, so called primary recovery in which fluid movement to the surface under its own initiative without any type of formation treatment is possible. Supplemental recovery is necessary when any of these basic elements is missing or when drive energy has been depleted by primary recovery. Frequently it is necessary to apply corrective treatment in order to reduce the petroleum viscosity and simultaneously supply fluid drive energy.
The most extreme example of petroleum-containing formations which require a substantial amount of treatment to permit the recovery of petroleum therefrom are the so called bituminous sands or oil sand deposits. Extensive oil sand deposits are found in the western United States, in northern Alberta Canada, and in Venezuela, and lesser deposits are located in Europe and Asia. The Athabasca deposits in Alberta Canada are the most famous, and it is estimated that these deposits contain as much as 700 billion barrels of petroleum. Some production has been obtained from shallow deposits by strip mining, but most of the deposits are located at depths too great to permit strip mining by current technology. The fluid permeability of tar sand deposits in their initial state is quite low and the viscosity of the bituminous petroleum contained therein is in the range of millions of centipoise at formation temperatures. Accordingly, substantial treatment is necessary to reduce the viscosity of the bituminous petroleum contained in these oil sand deposits in order to accomplish substantial flow of petroleum through the formation to the wells completed therein, even if adequate pressure differential is applied between an injection well and the production well.
Several methods have been described in the literature for recovering bituminous petroleum from oil sand deposits. Most of these methods employ steam injection, either alone or in combination with emulsifying chemicals such as caustic solution, or in combination with hydrocarbons. While the technical feasibility of processes for recovering viscous bituminous petroleum from oil sand deposits involving steam injection has been demonstrated, none of the processes have been developed to a commercially viable level. Enormous quantities of steam are required to reduce the viscosity of bituminous petroleum to a level such that it will flow through the formation to the production well from which it can be pumped or lifted to the surface, and the fuel cost for generating steam, especially superheated steam, is very substantial for a number of reasons. The most desirable fuels for use in firing generators and boilers to generate steam for use in thermal in situ recovery techniques employing steam injection include natural gas and relatively low molecular weight fuel oils including diesel oil, and these materials are in short supply and have become quite expensive because of their desirability for use in residential and industrial heating and other purposes. Accordingly, there is a significant need for a method of generating steam for separating highly viscous petroleum including bituminous petroleum found in oil sand deposits using less expensive and less desirable fuels than natural gas and relatively low molecular weight liquid hydrocarbon fuels.
Another significant cost associated with the generation of steam suitable for use in steam injection is the cost of treating the vast quantities of water required for the generation of the steam. Water must be treated to remove suspended particulate matter such as silt, clay, etc. as well as dissolved salt, prior to introduction of feed water into a conventional steam generator or boiler. Failure to properly treat feed water prior to its use in generating steam can result in rapid buildup of scale and other deposits on the tubes found in conventional steam generation equipment, and concentration of dissolved materials in the liquid fraction of saturated steam such as is commonly utilized in steam injection processes. In many of the regions where viscous petroleum formations are found and where it would be desirable to apply steam injection techniques for stimulating production thereof, the most readily available and least expensive water supplies include water from rivers, lakes, or water produced from oil producing formations in connection with other oil producing operations, which are frequently very high in suspended particulate matter content and in dissolved solids content. Excessively costly treatment is required to use such feed water in conventional steam generating equipment. Accordingly, it can be appreciated that there is a significant need for a method for generating steam using relatively dirty water without the costly treatment processes as are ordinarily needed.